I just measured 1.36 MB total transfer for a full reload on GitHub.com. Five avatars were cached in memory; their total size was under 1 MB.

I'm using Safari 14.0. Do you have any measurements (maybe on another browser) that show GitHub serving particularly large pages?

It was just a cheap dig, but try combining this page https://github.com/ValveSoftware/Proton/issues/2907 with this extension https://github.com/findepi/user.js/blob/master/github-expand... to expand all of the missing comments. You'll get a lot of xfer for what is effectively just some plaintext.

A good chunk of the data transferred in that issue for me are images, not plaintext. Lots of screenshots of the videogame.

Okay, that is a valid point. There's clearly a lot of unnecessary overhead with the expand button in issues.